Did you read the article you posted?
"We believe that this isn't the time to make large scale migration to a new technology. We will focus our Cassandra work on new projects that we wouldn't be able to ship without a large-scale data store.
"We're investing in Cassandra every day. It'll be with us for a long time and our usage of it will only grow."
As said in this post Twiter stop working on using Cassandra as a store for Tweets, but there nothing said why they made this decision? Does anybody have mo information